Astronomy for Beginners 6: Lunar Eclipses

By Duncan Lunan.

When the Moon eclipses the Sun, its shadow follows a narrow track across the Earth’s surface and few people see what happens;  but when the Earth’s shadow falls on the Moon, it can be seen from anywhere on the hemisphere facing the Moon at the time.

In the early part of the eclipse, often there’s little or nothing to be seen.  That’s because the Sun shows as a disc half a degree across, and it takes time for the Earth to cover it. During that time, from the Moon, the Sun would be seen only partly eclipsed.  This outer part of the Earth’s shadow is known as the penumbra.  From within the inner cone, the umbra, the Sun would seem to be completely eclipsed.  The curved edge of the umbra, as it sweeps across the Moon, helped to convince the ancient Greeks that the world must be round.

As it happens, the Moon and the Sun appear almost the same size from here, and that’s why solar eclipses are so dramatic when the outer atmosphere of the Sun flashes into view.  (It took quite a long time to establish that it was the Sun’s atmosphere and not the Moon’s.)  But from the Moon, the Earth has four times the apparent size of the Sun – another reason why lunar eclipses are more frequent and last longer.

Nobody has seen a lunar eclipse from a lunar viewpoint, though artists have been fascinated by the prospect for a long time.  One of the earliest pieces of astronomical art I know is the colour page in the 19th century book “The Moon”, by Naysmith & Carpenter, showing the red-rimmed Earth eclipsing the Sun.  The late Chesley Bonestell had several paintings on this theme, and it had been hoped that they could be compared with the reality by the camera on the Apollo 15 Lunar Rover, after the astronauts returned to Earth.   Unfortunately contact was lost before the due eclipse occurred – the British artist David Hardy painted the Rover being driven away by a mocking alien.

Nevertheless we have one ‘picture’ of the phenomenon:  compiled with some difficulty by a sensor on the unmanned spacecraft Surveyor 3 before manned landings on the Moon.  The surprising thing was that the Earth wasn’t surrounded by a uniform ring of light – there were breaks in it, like the ‘Bailly’s Beads’ effect caused by mountains on the rim of the Moon during eclipses seen from here.  Earth has no mountains of such size in relation to its diameter, but careful checking revealed that the breaks in the ring coincided with masses of cloud, on the rim of the Earth seen from the Moon.

What this means is that we can never be entirely sure what we’re going to see in a lunar eclipse.  How soon the penumbral effect will become visible, and how dark the umbra will be, depends very much on the clouds round the sunrise and sunset limbs of the Earth at the time.

Clouds have another marked effect on eclipses, as far as we’re concerned.  Back in my schooldays, before Luna III showed us the far side of the Moon for the first time, I was brought up on a poem – attributed to a servant of a poet, in the 30s, by Patrick Moore – which ran:

“Oh Moon, lovely Moon, with the beautiful face,

Careering along through the bound’ries of space;

Whenever I see you, I think in my mind,

Shall I ever, oh ever, behold thy behind?”

Not often quoted nowadays, when the Farside has been thoroughly mapped.  But one night back then, while waiting out an eclipse in which the clouds never broke at all, I came up with my own version.

“Oh Moon, lovely Moon, with the unknown behind,

On this night of eclipse how I think in my mind,

As penumbra and umbra fall on you through space,

I would be quite content with your Earth-shadowed face!”

Still, we can watch in hope…

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